Preparation method for magnesium matrix composite

ABSTRACT

The invention relates to a preparation method for a magnesium matrix composite. The preparation method comprises the following steps: (1) preparing magnesium ingots as raw materials and salt flux and reinforcements; (2) placing the salt flux in a crucible, performing heating to prepare salt flux melts, adding the reinforcements; (3) performing pouring into a normal-temperature crucible, and performing cooling to obtain precursors; (4) adding the raw materials in an iron crucible, and performing melting at 953K-1043K; (5) placing the precursors in raw material melt, after stirring, under a condition of 953K-993K, performing standing so that scum and melt are obtained; and (6) removing the scum, lowering temperature to 973K-982K, and performing casting. The method provided by the present invention is simple in process and low in cost. The method can be used for preparing bulk structural members of the magnesium matrix composite, and can be used for automatic production.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a preparation method for composites, in particular to a preparation method for a magnesium matrix composite.

2. The Prior Arts

Magnesium alloys have the advantages of being low in density, high in specific strength, excellent in vibration-damping performance, electromagnetic shielding performance and machinability, and the like; the magnesium alloys are an ideal material adopting a lightweight structure; and research and application of the magnesium alloys are highly valued in recent years. However, the magnesium alloys also have the shortcomings of being high in melt casting difficulty, difficult in plastic deformation, poor in high-temperature creep resistance, poor in corrosion resistance and the like, wherein the low strength and easy occurrence of yield deformation restrict the application of the magnesium alloys; the magnesium alloys are generally applied to secondary load-bearing components only, which restricts the application field of the magnesium alloys severely; and therefore, it is urgent to produce the lightweight magnesium matrix composite having the advantages of being low in cost and high in performance.

Compared with traditional magnesium and its alloys, the magnesium matrix composite has some special performances and other excellent comprehensive performances except for excellent mechanical properties; and at present, methods for preparing a reinforced magnesium matrix composite mainly comprise a traditional mechanical stirring casting method, a squeeze casting method, an injection molding method, an in-situ composite method, and the like.

In the traditional mechanical stirring casting method, particles, whiskers, fibers and other reinforcements are added to molten metal melt, and the reinforcements are uniformly distributed in matrices by a mechanical stirring method. The traditional mechanical stirring casting method has the advantages of being low in cost and simple in technological process, and can be used for batch production and bulk production, and can be widely applied to the industries, such as aerospace and automobile manufacturing. The key problem for preparing the magnesium matrix composite is how to distribute the reinforcements in the metal melt uniformly; however, most of the reinforcements are often aggregated or precipitated when entering into the molten metal melt, and accordingly are hard to disperse in the metal melt uniformly; in the stirring process, gas impurities can be mixed along with stirring, and the melt viscosity can be increased by reinforcement particles, so that gas is hard to escape; and therefore, there are very high requirements for mechanical stirring. The reinforcements generate such phenomenon in the melt that mainly because of density difference between the reinforcements and metal, gravity segregation certainly occurs; and because of the poor wettability of the reinforcements for liquid metal, the reinforcements cannot be dispersed in the matrices properly.

The squeeze casting method is a precision casting method used for mold filling and solidification of the liquid metal or semisolid metal under high pressure; firstly, the reinforcements are preformed and heated, and then, molten metal or melt is poured, and the reinforcements are pressed with molds, and cooled to obtain composite castings. By the squeeze casting method, influence of the gas impurities on product quality can be reduced, and low requirements on wettability have been achieved; compact and uniform castings can be prepared, and the volume fraction of the reinforcements which can be added is also increased to 30%-50%; so that the performance of the composite can be improved obviously. However, the problem of influence of pressure on the casting quality exists; under high pressure, molten magnesium can generate turbulent flow, resulting in such phenomena of magnesium oxidization and gas hold-up; and under low pressure, a part of gas cannot be removed, resulting in the phenomenon that the castings are not compact. In addition, the squeeze casting method cannot be used for production of bulk castings or automatic batch production.

In the injection molding method, molten metal is atomized by using rare gas and sprayed, mixed with the reinforcements conveyed by the rare gas at the other end, and deposited and cooled on a platform to obtain composite parts. The injection molding method applies a metal rapid solidification technology, and restrains growth of grains and formation of segregation, so that the gains are refined, and the reinforcements are distributed uniformly. Metal atomization and hybrid deposition are two major influence factors of the injection molding method; in the metal atomization process, the parts often have higher porosity and shrinkage phenomenon along with gas transfer; if solidification rate is too fast after deposition, the reinforcements and the matrix are poor in compound effect, or even are not compounded; if the solidification rate is slow, the reinforcements can be distributed unevenly, or even segregated; and the injection molding method, as a novel composite preparation method, is high in cost, and therefore, is not applicable for automatic batch production.

The in-situ composite method is a novel method for preparing metal matrix composite; in this method, direct adding of the reinforcements is not required, but the reinforcements are generated in the melt by chemical reaction or other special reactions; and nucleation and growth are both finished in the matrix, and therefore, the phenomenon of incompatibility and poor combination between reinforcements and matrix do not occur, influence of wettability is avoided, resulting in the composite is uniform and pure. The method is low in cost and simple in technological process, and the obtained part is high in quality; and however, the method for generating the reinforcements by chemical reaction has limitations of less reinforcements, so the requirements of batch production cannot be obtained.

In a powder metallurgy method, metal powder and reinforcement powder are mixed by ball milling, and then formed by hot-press sintering under the vacuum conditions. By using the powder metallurgy method, matrix alloys do not need to be heated to a molten state, so that the interface reaction between matrix and reinforcements can be prevented; and after mixing, the reinforcements are distributed in the matrix uniformly, and achieve a favorable effect of strengthening. However, due to significant difference in size, shape and performance between the reinforcements and the matrix alloys, compared with the interface bonding strength of the composite produced by the casting method, the interface bonding strength of the composite can be reduced after combination. In addition, the powder metallurgy method is not applicable for larger structural materials, but applicable for functional materials of small parts; the technological process of the powder metallurgy method is relatively complicated, and high in cost; and problems exist in a transportation process. Therefore, the powder metallurgy method greatly restricts preparation and production of the magnesium matrix composite as a structural material.

For selection of the reinforcements, whether the wettability between the reinforcements and the matrix is better or not, whether the interface bonding strength is proper or not, and whether chemical reaction occurs at the interfaces or not, need to be noticed. At present, the reinforcements are approximately classified into three categories: whiskers, fibers and particles, such as lanthanum oxide particles, cerium oxide particles, silicon carbide whiskers and carbon fibers, wherein, the fiber reinforcements are high in cost, and can form strong textures, so that the composite has poor performance, and the whiskers and particles reinforced magnesium matrix composite have the advantages of being easy to machine, stable in size, and the like. The rare earth oxide particle reinforcements are high in melting point, cannot be molten after being added to magnesium or magnesium alloy solutions, and besides, cannot produce chemical reaction with the matrix; if the reinforcements can exist in the matrix uniformly, segregation of interstitial impurities at the grain boundary is reduced, and the grain boundary strength can be improved; in addition, rare earth oxides achieve the effect of pinning for dislocation, and prevent dislocation from moving, so that the strength of magnesium alloy is improved, and the plasticity cannot be reduced greatly; and however, if the rare earth oxides are directly added to the matrix melt, the particles can be agglomerated due to poor wettability, and cannot be dispersed in the matrix properly, so that the effect of dispersion strengthening is not achieved.

SUMMARY OF THE INVENTION

The present invention aims to provide a preparation method for a magnesium matrix composite. Reinforcements are dispersed with salt flux, so that surface wettability is improved; then, the reinforcements are added to a magnesium melt, so that the problem of wettability between the reinforcements and a matrix is solved; and the strength of the magnesium matrix composite is improved while the process is simplified.

The preparation method comprises the following steps:

(1) Preparing magnesium ingots as raw materials; preparing salt flux and reinforcements, wherein the salt flux is a mixture of barium chloride, magnesium chloride, sodium chloride and calcium chloride, the barium chloride accounts for 35-50% of a total mass of the salt flux, the magnesium chloride accounts for 10-20% of a total mass of the salt flux, the sodium chloride accounts for 10-20% of a total mass of the salt flux, a balance is the calcium chloride and impurities, the impurities account for no more than 1% of a total mass of the salt flux, the reinforcements are elementary metal, rare earth oxides, carbides, borides or metal oxides, the elementary metal is W, Mo or Ni, the rare earth oxides are La₂O₃, CeO₂ or Y₂O₃, the carbides are TiC or SiC, the borides are ZrB₂, the metal oxides are MgO or SiO₂, the reinforcements are 0.1-30% of a total volume of the raw materials, and the reinforcements are 1-50% of a total volume of the salt flux;

(2) Placing the salt flux in a clay crucible or a graphite crucible, performing heating to 773K-923K to prepare salt flux melts, placing the reinforcements in the salt flux melts, and performing stirring until the reinforcements are uniformly dispersed to prepare a liquid-solid mixture;

(3) Pouring the liquid-solid mixture into a normal-temperature clay crucible or graphite crucible, and performing cooling to normal temperature to obtain precursors;

(4) Preheating an iron crucible until a body of the iron crucible is in a dark red heat, then placing the raw materials in the iron crucible, and performing melting on the raw materials at 953K-1043K to form a raw material melt;

(5) Placing the precursors in the raw material melt of 953K-1043K, performing stirring until the precursors are dispersed uniformly, under a condition of 953K-993K, adding refining agents, performing stirring for refining, after the refining is finished, controlling temperature to 1013K-1023K, and performing standing so that impurity components are separated from composite components, and scum and composite melt are obtained;

(6) Removing the scum from a surface of the composite melt, then cooling the composite melt to 973K-982K, and performing casting to form the magnesium matrix composite ingots.

A purity of the magnesium ingots is greater than or equal to 99.85%.

The reinforcements are fibers, particles or whiskers, wherein a particle size of the particles is 300 nm-20 μm; a diameter of the whiskers is 0.1 μm-1 μm, and the length is 10 μm-100 μm; and a diameter of the fibers is 5 μm-20 μm, and a continuous length is 10 mm-70 mm.

In the step (5), the precursors are crushed into particles with the particle size of no more than 5 cm, and then the particles are placed in the raw material melt.

In the step (2), a stirring rate is 100 r/min-200 r/min, and a time is 2 min-10 min.

In the step (5), a stirring rate is 100 r/min-300 r/min, and a time is 5 min-15 min.

In the step (2), when the reinforcements are added to the salt flux melts, all the reinforcements are added in three to five times, and the adding amount each time accounts for less than 50% of the total mass of the reinforcements.

In the step (5), before refining, materials in the iron crucible are degassed by using mixed gas, and the mixed gas is formed by mixing components in percentage by volume of 0.2%-0.3% of sulfur hexafluoride, 25%-50% of carbon dioxide and balance air.

In the step (5), a standing time is 10 min-30 min.

In the step (1), the magnesium ingots and other metal components are prepared as the raw materials, when the step (4) is performed, the magnesium ingots and the other metal components are placed in the iron crucible jointly, melting is performed, and stirring and uniform mixing are performed to form a raw material melt, wherein the other metal components are one or more of aluminum ingots, zinc ingots, manganese chloride, magnesium-rare earth alloys, magnesium-zirconium alloys and magnesium-silicon alloys, and aluminum, zinc, manganese, rare earth, zirconium and silicon in the other metal components account for no more than 10% of a total mass of the raw materials.

In the step (4), covering flux is scattered onto a surface of the raw material melt so as to prevent magnesium from burning, wherein the covering flux is No. 2 flux; when the step (5) is performed, the covering flux is mixed with the scum; and when the step (6) is performed, the covering flux and the scum are removed together.

In the step (5), the refining agents are No. 2 flux.

Raw material components in the magnesium matrix composite account for 80-99.9% of the total volume, and the components of the reinforcements account for 0.1-20% of the total volume.

The preparation method is characterized in that the reinforcements are put into molten salt flux, and uniformly dispersed in molten salt by mechanical stirring; the surface wettability of the reinforcements is improved by utilizing the high wettability between the reinforcements and the molten salt; since the difference between the density of barium chloride in the selected molten salt and the density of magnesium melt is larger, the reinforcements are separated from the molten salt after being added to the magnesium melt; the wettability between the reinforcements and the magnesium melt is high after surface modification of the reinforcements, and the reinforcements can be dispersed in the magnesium melt uniformly; the melt can be refined by the used salt flux effectively, and impurities and a covering melt are removed; and the magnesium is prevented from being overburnt. The wettability of the reinforcements can be improved by the barium chloride and other salts, so that the reinforcements are easy to disperse in the matrix uniformly; the method provided by the present invention is simple in process and low in cost, and the strength of the magnesium matrix composite can be improved greatly; and the method can be used for preparing bulk structural members of the magnesium matrix composite, can be used for automatic production, and is of great significance in development of magnesium industry.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an SEM diagram of a lanthanum oxide reinforced magnesium matrix composite in the embodiment 1 of the present invention; in the figure, (b) is the partial enlarged view of (a);

FIG. 2 is an XRD diagram of the lanthanum oxide reinforced magnesium matrix composite in the embodiment 1 of the present invention; in the figure, (a) is the diffraction peaks of La₂O₃, and (b) is the diffraction peaks of magnesium matrix composite;

FIG. 3 is an SEM diagram of cerium dioxide reinforced magnesium matrix composite in the embodiment 2 of the present invention; in the figure, (b) is the partial enlarged view of (a).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A detailed description of the preparation method is given below in combination with the embodiments of the present invention.

In the embodiments of the present invention, temperature is measured with a thermocouple to ensure the measurement accuracy of the temperature.

The purity of an aluminum ingot and a zinc ingot in the present invention is 98.9%-99.9%.

Manganese chloride in the present invention is technical pure.

Magnesium-rare earth alloys, magnesium-zirconium alloys and magnesium-silicon alloys in the present invention are collectively referred to as master alloys, and rare earth, zirconium and silicon in the master alloys account for 10%-40% of the total mass of the master alloys separately.

The aluminum ingot, the reinforcements and No. 2 flux adopted in the embodiments of the present invention are products purchased on the market.

Barium chloride, magnesium chloride, sodium chloride and calcium chloride adopted in the embodiments of the present invention are industrial grade products purchased by commercial.

An electron microscope adopted in the embodiments of the present invention is Shimadzu SSX550.

X-ray diffraction observation equipment in the embodiments of the present invention is PANalytical B. V. X pertpro.

The mass percentage of the reinforcements of the magnesium matrix composite in the embodiments of the present invention is analyzed and calculated by using an X-ray fluorescence spectrum, and then converted into volume percentage.

The purity of the magnesium ingot in the embodiments of the present invention is greater than or equal to 99.85%.

The reinforcements in the embodiments of the present invention are fibers, particles or whiskers, wherein the particle size of the particles is 300 nm-20 μm; the diameter of the whiskers is 0.1 μm-1 μm, and the length is 10 μm-100 μm; and the diameter of the fibers is 5 μm-20 μm, and the continuous length is 10 mm-70 mm.

Before refining in the embodiments of the present invention, materials in the iron crucible are degassed by using mixed gas, and the mixed gas is formed by mixing components in percentage by volume of 0.2%-0.3% of sulfur hexafluoride, 25%-50% of carbon dioxide and balance air, and inflation time of the mixed gas is 2 min-5 min.

The adding quantity of refining agents in the embodiments of the present invention is 0.5%-0.8% of the total mass of all melts in the iron crucible.

Embodiment 1

A magnesium ingot is prepared as raw materials; salt flux and reinforcements are prepared; the salt flux is a mixture of barium chloride, magnesium chloride, sodium chloride and calcium chloride, wherein the barium chloride accounts for 45% of the total mass of the salt flux; the magnesium chloride accounts for 20% of the total mass of the salt flux, and the sodium chloride accounts for 15% of the total mass of the salt flux; the balance is the calcium chloride and impurities, and the impurities account for no more than 1% of the total mass of the salt flux; the reinforcements are rare earth oxides, namely La₂O₃ particles; the reinforcements account for 0.5% of the total volume of the raw materials; the reinforcements account for 3% of the total volume of the salt flux;

The salt flux is placed in a clay crucible, and heated to 803K to form salt flux melts; the reinforcements are added to the salt flux melts, and uniformly dispersed by stirring to form a liquid-solid mixture; the stirring rate is 100 r/min, and the stirring time is 10 min; when the reinforcements are added to the salt flux melts, all the reinforcements are added in three times, and the adding amount each time accounts for less than 50% of the total mass of the reinforcements;

The liquid-solid mixture is poured into the normal-temperature clay crucible, and cooled to normal temperature to obtain precursors;

An iron crucible is preheated until the body of the iron crucible is in a dark red heat, then the raw materials are placed in the iron crucible, and melting is performed on the raw materials at 973K to form a raw material melt; the covering flux is scattered onto the surface of the raw material melt, and used for preventing the magnesium from burning, and the covering flux is the No. 2 flux;

Firstly, the precursors are crushed into particles with the particle size of no more than 5 cm, then placed in the raw material melt of 973K, and stirred until the precursors are dispersed uniformly; next, refining agents are added at 973K, and stirred for refining; the refining agents are the No. 2 flux, the stirring rate is 100 r/min, the stirring time is 15 min, and the temperature rises to 1013K after completion of refining; the precursors stand so that impurity components are separated from composite components to obtain scum and a composite melt; the standing time is 30 min;

The scum is removed from the surface of the composite melt, and then the composite melt is cooled to 973K, and cast to form the magnesium matrix composite; and the reinforcement components in the magnesium matrix composite account for 0.41% of the total volume, and the balance is the raw material components.

The SEM diagram of the magnesium matrix composite (lanthanum oxide reinforced magnesium matrix composite) is shown as FIG. 1, the XRD diagram is shown as FIG. 2, and as shown in the figures, La₂O₃ phases are uniformly distributed in the matrix.

Under the same conditions, the quantity of the reinforcements is regulated for parallel test, and the reinforcements are respectively 1%, 3%, 5%, 7%, 9%, 15% and 20% of the total volume of the raw materials; and in the lanthanum oxide reinforced magnesium matrix composite which is formed finally, 80%-90% of the total mass of the reinforcements are reserved in the matrix.

Embodiment 2

The method is the same as that in the embodiment 1, the differences include:

(1) In salt flux, barium chloride accounts for 50% of the total weight of the salt flux, magnesium chloride accounts for 10% of the total weight of the salt flux, and sodium chloride accounts for 20% of the total weight of the salt flux;

(2) Reinforcements are rare-earth oxides, namely CeO₂ particles;

(3) The reinforcements are 1% of the total volume of raw materials, and are 5% of the total volume of the salt flux;

(4) The salt flux is placed in a graphite crucible, and heated to 773K to form salt flux melts; the stirring rate is 200 r/min, and the stirring time is 2 min; when the reinforcements are added to the salt flux melts, all the reinforcements are added in four times;

(5) A liquid-solid mixture is poured into the normal-temperature graphite crucible;

(6) Raw materials in the iron crucible are molten at 953K to form raw material melt;

(7) Precursors are placed in the raw material melt at 953K after being crushed, refining agents are added at 953K, and stirred for refining, the stirring rate is 300 r/min, the stirring time is 5 min, the temperature rises to 1023K after completion of refining, and the standing time is 10 min;

(8) The composite melt is cooled to 982K, and cast to form the magnesium matrix composite; and the reinforcement components in the magnesium matrix composite account for 0.85% of the total volume, and the balance is the raw material components.

The SEM diagram of the magnesium matrix composite (lanthanum oxide reinforced magnesium matrix composite) is shown as FIG. 3, and CeO₂ phases are uniformly distributed in the matrix.

Embodiment 3

The method is the same as that in the embodiment 1, the differences include:

(1) Magnesium ingots and other metal components are prepared as the raw materials, wherein the other metal components are aluminum ingots and account for 5% of the total mass of the raw materials; in salt flux, barium chloride accounts for 35% of the total weight of the salt flux, magnesium chloride accounts for 15% of the total weight of the salt flux, and sodium chloride accounts for 10% of the total weight of the salt flux;

(2) Reinforcements are borides namely ZrB₂;

(3) The reinforcements are 10% of the total volume of raw materials, and are 15% of the total volume of the salt flux;

(4) The salt flux is placed in a graphite crucible, and heated to 883K to form salt flux melts; the stirring rate is 150 r/min, and the stirring time is 5 min; when the reinforcements are added to the salt flux melts, all the reinforcements are added in five times;

(5) A liquid-solid mixture is poured into the normal-temperature graphite crucible;

(6) The magnesium ingots and other metal components are placed in the iron crucible together, and the raw materials in the iron crucible are molten at 1043K to form raw material melt;

(7) Precursors are placed in the raw material melt at 1043K after being crushed, refining agents are added at 1043K, and stirred for refining, the stirring rate is 200 r/min, the stirring time is 10 min, the temperature is cooled to 1013K after completion of refining, and the standing time is 20 min;

(8) The composite melt is cooled to 978K, and cast to form the magnesium matrix composite; and the reinforcement components in the magnesium matrix composite account for 8.1% of the total volume, and the balance is the raw material components.

Embodiment 4

The method is the same as that in the embodiment 1, the differences include:

(1) Magnesium ingots and other metal components are prepared as the raw materials, wherein the other metal components are zinc ingots and account for 2% of the total mass of the raw materials;

(2) Reinforcements are elementary metal W;

(3) The reinforcements are 15% of the total volume of raw materials, and are 25% of the total volume of the salt flux;

(4) The salt flux is placed in a graphite crucible, and heated to 923K to form salt flux melts, the stirring rate is 120 r/min, and the stirring time is 8 min;

(5) A liquid-solid mixture is poured into the normal-temperature graphite crucible;

(6) The magnesium ingots and other metal components are placed in the iron crucible together, and the raw materials in the iron crucible are molten at 1043K to form raw material melt;

(7) Precursors are placed in the raw material melt at 1043K after being crushed, refining agents are added at 1043K, and stirred for refining, the temperature is cooled to 1018K after completion of refining, and the standing time is 15 min;

(8) The composite melt is cooled to 980K, and cast to form the magnesium matrix composite; and the reinforcement components in the magnesium matrix composite account for 13.3% of the total volume, and the balance is the raw material components.

Embodiment 5

The method is the same as that in the embodiment 1, the differences include:

(1) Magnesium ingots and other metal components are prepared as the raw materials, wherein the other metal components are magnesium-rare earth alloys and account for 4% of the total mass of the raw materials; in salt flux, barium chloride accounts for 40% of the total weight of the salt flux, magnesium chloride accounts for 20% of the total weight of the salt flux, and sodium chloride accounts for 20% of the total weight of the salt flux;

(2) Reinforcements are carbides namely TiC;

(3) The reinforcements are 22% of the total volume of raw materials, and are 40% of the total volume of the salt flux;

(4) The salt flux is placed in a graphite crucible, and heated to 828K to form salt flux melts; the stirring rate is 180 r/min, and the stirring time is 3 min; when the reinforcements are added to the salt flux melts, all the reinforcements are added in four times;

(5) A liquid-solid mixture is poured into the normal-temperature graphite crucible;

(6) The magnesium ingots and other metal components are placed in the iron crucible together, and the raw materials in the iron crucible are molten at 988K to form raw material melt;

(7) Precursors are placed in the raw material melt at 988K after being crushed, refining agents are added at 988K, and stirred for refining, the stirring rate is 300 r/min, the stirring time is 5 min, the temperature rises to 1023K after completion of refining, and the standing time is 25 min;

(8) The composite melt is cooled to 979K, and cast to form the magnesium matrix composite; and the reinforcement components in the magnesium matrix composite account for 18.6% of the total volume, and the balance is the raw material components.

Embodiment 6

The method is the same as that in the embodiment 1, the differences include:

(1) Magnesium ingots and other metal components are prepared as the raw materials, wherein the other metal components are magnesium-zirconium alloys and magnesium-silicon alloys, and zirconium and silicon account for 10% of the total mass of the raw materials; in salt flux, barium chloride accounts for 50% of the total weight of the salt flux, magnesium chloride accounts for 10% of the total weight of the salt flux, and sodium chloride accounts for 10% of the total weight of the salt flux;

(2) Reinforcements are metal oxides namely SiO₂;

(3) The reinforcements are 26% of the total volume of raw materials, and are 45% of the total volume of the salt flux;

(4) Heating is performed to 873K to form salt flux melts, the stirring rate is 160 r/min, the stirring time is 4 min, and when the reinforcements are added to the salt flux melts, all the reinforcements are added in five times;

(5) The magnesium ingots and other metal components are placed in the iron crucible together, and the raw materials in the iron crucible are molten at 993K to form raw material melt;

(6) Precursors are placed in the raw material melt at 993K after being crushed, refining agents are added at 993K, and stirred for refining, the stirring rate is 200 r/min, the stirring time is 10 min, the temperature rises to 1013K after completion of refining, and the standing time is 25 min;

(7) The composite melt is cooled to 976K, and cast to form the magnesium matrix composite; and the reinforcement components in the magnesium matrix composite account for 21.1% of the total volume, and the balance is the raw material components. 

What is claimed is:
 1. A preparation method for a magnesium matrix composite, comprising: (1) preparing magnesium ingots as raw materials; preparing salt flux and reinforcements, wherein the salt flux is a mixture of barium chloride, magnesium chloride, sodium chloride and calcium chloride, the barium chloride accounts for 35-50% of a total mass of the salt flux, the magnesium chloride accounts for 10-20% of a total mass of the salt flux, the sodium chloride accounts for 10-20% of a total mass of the salt flux, a balance is the calcium chloride and impurities, the impurities account for no more than 1% of the total mass of the salt flux, the reinforcements are elementary metal, rare earth oxides, carbides, borides or metal oxides, the elementary metal is W, Mo or Ni, the rare earth oxides are La₂O₃, CeO₂ or Y₂O₃, the carbides are TiC or SiC, the borides are ZrB₂, the metal oxides are MgO or SiO₂, the reinforcements are 0.1%-30% of a total volume of the raw materials, and the reinforcements are 1%-50% of a total volume of the salt flux; (2) placing the salt flux in a clay crucible or a graphite crucible, performing heating to 773K-923K to prepare salt flux melts, placing the reinforcements in the salt flux melts, and performing stirring until the reinforcements are uniformly dispersed to prepare a liquid-solid mixture; (3) pouring the liquid-solid mixture into a normal-temperature clay crucible or graphite crucible, and performing cooling to normal temperature to obtain precursors; (4) preheating an iron crucible until a body of the iron crucible is in a dark red heat, then placing the raw materials in the iron crucible, and performing melting on the raw materials at 953K-1043K to form a raw material melt; (5) placing the precursors in the raw material melt of 953K-1043K, performing stirring until the precursors are dispersed uniformly, under a condition of 953-993K, adding refining agents, performing stirring for refining, after the refining is finished, controlling temperature to 1013K-1023K, and performing standing so that impurity components are separated from composite components, and scum and a composite melt are obtained; (6) removing the scum from a surface of the composite melt, then cooling the composite melt to 973-982K, and performing casting to form the magnesium matrix composite.
 2. The preparation method according to claim 1, wherein a purity of the magnesium ingots is greater than or equal to 99.85%.
 3. The preparation method according to claim 1, wherein the reinforcements are fibers, particles or whiskers, wherein a particle size of the particles is 300 nm-20 μm, a diameter of the whiskers is 0.1 μm-1 μm, a length is 10 μm-100 μm, a diameter of the fibers is 5 μm-20 μm, and a continuous length is 10 mm-70 mm.
 4. The preparation method according to claim 1, wherein in the step (2), a stirring rate is 100 r/min-200 r/min, and a time is 2 min-10 min.
 5. The preparation method according to claim 1, wherein in the step (5), a stirring rate is 100 r/min-300 r/min, and a time is 5 min-15 min.
 6. The preparation method according to claim 1, wherein in the step (5), a standing time is 10 min-30 min.
 7. The preparation method according to claim 1, further comprising in the step (1): preparing the magnesium ingots and other metal components as the raw materials; when the step (4) is performed, placing the magnesium ingots and the other metal components in the iron crucible jointly, performing melting, and preforming stirring and uniform mixing to form a raw material melt, wherein the other metal components are one or more of aluminum ingots, zinc ingots, manganese chloride, magnesium-rare earth alloys, magnesium-zirconium alloys and magnesium-silicon alloys, and aluminum, zinc, manganese, rare earth, zirconium and silicon in the other metal components account for no more than 10% of a total mass of the raw materials.
 8. The preparation method according to claim 1, further comprising in the step (4): enabling covering flux to be scattered onto a surface of the raw material melt so as to prevent magnesium from burning, wherein the covering flux is No. 2 flux; when the step (5) is performed, mixing the covering flux with the scum; and when the step (6) is performed, removing the covering flux and the scum together.
 9. The preparation method according to claim 1, wherein in the step (5), the refining agents are the No. 2 flux.
 10. The preparation method according to claim 1, wherein raw material components in the magnesium matrix composite account for 80-99.9% of a total volume, and components of the reinforcements account for 0.1-20% of the total volume. 